The field of the invention is diagnosis, prophylaxis, and treatment of gastric disease and nickel-related disorders; and non-clinical nickel detoxification The invention also relates to the bacterium Helicobacter pylori.
The bacterium Helicobacter pylori was first isolated from human gastric mucosa in 1983, and was originally identified as a member of the genus Campylobacter (either C. pylori or C. pyloridis; Warren and Marshall, Lancet i:1273, 1983; Marshall and Goodwin, Int. J. Syst. Bacteriol. 37:68, 1987). H. pylori is recognized as a pathogen, and is a major cause of chronic gastritis, inflammation of the gastric mucosa, and peptic ulcers. It can also contribute to the development of gastric cancer (for review, see Sipponen et al., "Histology and Ultrastructure of Helicobacter pylori Infections: Gastritis, Duodenitis, and Peptic Ulceration, and Their Relevant Precancerous Conditions", in Helicobacter pylori: Biology and Clinical Practice 37, Goodwin and Worsley, eds., 1993).
H. pylori is able to survive in the highly acidic environment of stomach at least in part due to its high urease activity, which may raise the pH of the local environment by hydrolyzing endogenous urea into ammonia and carbon dioxide. The ammonia component affects the pH, and its local accumulation is thought to have a directly toxic effect on nearby mucosa.
Urease activity is one of the principal properties of the bacterium. The urease of H. pylori has been well characterized, and consists of a high molecular weight (550 kDa) multimeric enzyme. There are two primary subunits, UreA (66 kDa) and UreB (29.5 kDa), and these combine to make a larger subunit, six of which ultimately combine to form the intact protein. H. pylori urease enzyme is homologous to ureases of other bacterial species, and to plant ureases (e.g., jackbean).
All ureases studied to date contain nickel, and H. pylori urease has been shown to contain 5.21 nickel atoms per molecule. H. pylori urease activity depends on the availability of nickel ion in the enzyme active site; the cloned enzyme in E. coli that yields high levels of protein only has urease activity if a source of nickel ion (typically in the form of NiCl.sub.2) is provided in the E. coli growth medium. This nickel must be present during enzyme synthesis, and cannot be added after the protein is fully synthesized. Also, histidine or cysteine, amino acids that actively chelate nickel ions, prevent nickel uptake or entry into the cloned urease protein expressed in E. coli. Thus, recombinant urease activity is low if the cells are cultured in a rich medium which contains much histidine and cysteine.
No uptake mechanism for nickel by H. pylori or by the structural urease subunits has been defined. Accessory genes near the urease structural genes in several microorganisms have been postulated to involve nickel uptake, such as a UreD accessory gene product in Aspergillus nidulans, or the histidine-rich UreE gene products in Klebsiella aerogenes and P. mirabilis, but none of this has been proven. Separate mechanisms for uptake of nickel by the organism and incorporation of nickel by the urease may exist.